Update `ImportsContextCustomizer` so that whenever possible a more
specific cache key is used.

Prior to this commit the customizer would generate a key based on *all*
annotations on the test class. This has repeatedly caused issues where
test classes that should have the same cache key did not due to
unrelated annotations.

A new `DeterminableImports` interface has been added that can be
implemented by `ImportSelector` and `ImportBeanDefinitionRegistrar`
implementations that are able to determine their imports early. The
existing `ImportAutoConfigurationImportSelector` and
`AutoConfigurationPackages` classes have been retrofitted with
this interface.

Fixes gh-7953